1. Field of the Invention
This invention relates to digital linearisers for linearising swept frequency waveforms. It particularly, but not exclusively, relates to such waveforms for use in an FMCW radar.
2. Discussion of Prior Art
Digital frequency synthesisers utilising phase-locked loops are well-known in the art of frequency synthesis. Such frequency synthesisers generate highly accurate signals at regularly-spaced discrete frequencies. While a stepwise approximation to a swept waveform can be attained by progressively incrementing or decrementing through the frequency steps, the fact that frequency changes can only occur as discrete steps means that there will in general be a discrepancy between the waveform achievable in practice and an ideal FMCW waveform.
One known technique for decreasing step size is to use a variable modulus counter in conjunction with a conventional variable divider. Such an arrangement is shown in FIG. 1. A stable reference frequency f1 on line 11 from a reference frequency oscillator 1 is applied to a first input of a frequency/phase discriminator 2. The discriminator output on line 12 is passed through a loop filter 3 which both introduces the necessary phase shift required for loop stability and removes AC components from the discriminator output, thereby supplying a DC voltage on line 13 to a frequency control input of a voltage controlled oscillator (VCO) 4. The VCO output frequency is fed via line 14 to a variable modulus divider 6. The divider output on line 16 is fed to a programmable divider 5 whose output signal at frequency f2 is applied via line 15 to a second input of discriminator 2.
The variable modulus divider is switchable between two different division ratios such as 10 and 11 or 31 and 32, and in use is repetitively switched between its two states. Varying the ratio of time spent in each state allows smaller frequency steps to be obtained than if only the programmable divider were used.
Such an arrangement might be thought to be suitable for the production of swept frequencies, as the inherent quantisation error due to the use of discrete frequencies can in principle be made arbitrarily small by appropriately controlling the effective division ratio using dividers 5 and 6.
However, progressively decreasing the frequency step size requires the provision of a loop filter having a progressively longer time constant, making it impossible to generate rapidly-changing swept waveforms.
Attempting to generate swept waveforms by applying modulation to the frequency control input of the VCO will result in the generation of a swept frequency f2 which will of course be different from reference frequency f1 Within a 2 window, discriminator 2 will view the progressive change of frequency f2 as a progressively-changing phase error. Discriminator 2 will attempt to correct this perceived phase error, thereby tending to cancel out the modulation. Increasing the loop filter time constant to overcome this will result in unacceptably long response times when changing frequency ranges.